Fix nullable warnings when using Member() on nullable properties (#5191)

* Initial plan

* fix: use TMember? in Member/HasProperty selector parameter to fix nullable warnings

Change Expression<Func<TObject, TMember>> to Expression<Func<TObject, TMember?>>
in Member and HasProperty overloads so that selectors on nullable properties
(e.g. string?) no longer produce CS8603/CS8620/CS8619 warnings.

Co-authored-by: thomhurst <30480171+thomhurst@users.noreply.github.com>

* test: add MemberNullabilityTests covering reference types, non-null structs, nullable structs, enums, collections, and chaining

Co-authored-by: thomhurst <30480171+thomhurst@users.noreply.github.com>
Agent-Logs-Url: https://github.com/thomhurst/TUnit/sessions/abcf6a00-83b2-4c2d-be1a-acf22a4cf702

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: thomhurst <30480171+thomhurst@users.noreply.github.com>

Author: Copilot